Casters are known in the art for resiliently carrying heavy loads. The problem, however, is that for a caster to have resiliency, e.g., when encountering a horizontal force delivered when the support surface changes abruptly in height, the casters in the past have used energy absorbers which are sensitive to resonances caused by vertical loads. That is, the resiliency of the absorbers applies in the vertical direction as well, so that there are some frequencies under a vertical load that can induce harmonic oscillations. Examples of casters of this construction are shown in U.S. Pat. No. 2,738,542, where the casters are constructed to "give" under a sudden vertical load as well as a sudden horizontal load. Such harmonic oscillations created by vertical loads being "bounced", for example, are unacceptable when the load in question is a fine-tuned instrument. For example, clinical analyzers while transported by truck will not tolerate vertical harmonic oscillations due to bumps in the road.
Resistance to vertical loading is provided by conventional casters in frames having a solid vertical connection to the load itself. However, these are deficient in that they also totally lack resiliency in the horizontal direction, so that horizontal shock is either fully transmitted to the load, or the shock breaks the caster.
The problem has been to provide a caster that will resist oscillations caused by any sudden increases in vertical loading, and still provide resiliency against a sudden increase in horizontal force so as to protect the load from damage.
Although casters of the type shown in U.S. Pat. No. 2,442,831 might seem to provide resistance to vertical loading due to a direct vertical coupling through a stud, while providing a resilient damper to give resiliency under horizontal loading, such casters in fact are not free from susceptibility to vertical oscillations. One reason is that the long moment arm of the caster preloads the damper to the point that it has a significant harmonic resonance to induced vertical oscillations.